Uncharged, almost massless, and feeling only the weak interaction and gravity, neutrinos are the most elusive particles of the Standard Model. Although no less than 65 billion of them pass through every square centimetre of the Earth every second from the Sun alone, it took 25 years from when they were first postulated by Wolfgang Pauli before Fred Reines and Clyde Cowan managed to design an experiment to detect them. Yet without them, the Sun would not shine, most of the elements of the periodic table would not exist, and perhaps the Universe itself would be an empty wasteland.
In all the history of the Standard Model of particle physics, neutrinos have been the only sector of the model that has had to undergo serious revision. In the original formulation of the Standard Model, neutrinos were massless, came in three distinct types or flavours (electron, muon and tau), and were clearly distinct from their antiparticles. Now we know that neutrinos are not massless, and that the three flavours mix so that a neutrino produced as electron type can subsequently interact as muon or tau type. We do not know for sure that neutrinos and antineutrinos are not distinct, but instead are only distinguished by the direction of their spin—but theorists strongly believe this, and experimentalists are keeping an open mind.
In Sheffield, we study the way that neutrino flavours mix as the neutrinos travel from their birthplace, an effect known as neutrino oscillation. We are involved in three currently operating neutrino oscillation experiments—T2K, Super-Kamiokande and ANNIE—as well as several future projects: the Fermilab Short Baseline Neutrino Program, DUNE, and Hyper-Kamiokande.
A brief look at our neutrino programme
Our neutrino programme can be divided into two categories based on the detector technology: water Cherenkov experiments (coded blue below) and liquid argon experiments (coded grey).
T2K, Super-Kamiokande and Hyper-Kamiokande
A long-standing issue in neutrino oscillations is that there are a few signals that cannot be fitted into the standard picture of three neutrino flavours. The number of weakly interacting light neutrinos is firmly set to three by measurements of the Z boson width, so any additional neutrinos would have to be so-called sterile neutrinos that do not feel the weak interaction, and therefore can be detected only by their coupling—via neutrino oscillations—to the three active flavours. The Fermilab Short Baseline Neutrino Program is designed to confirm or refute these indications of non-standard neutrino oscillations with high precision. It consists of three liquid argon TPCs at different distances from the Fermilab Booster Neutrino Beam (BNB) target station, as shown below.
The intermediate and far detectors, MicroBooNE and ICARUS-T600, are repurposed from earlier experiments. The near detector, SBND, is a new build which will provide valuable experience for the similar technology of DUNE.
ANNIE and WATCHMAN
Alongside our main water Cherenkov neutrino programme based in Japan, we have two special-purpose projects, ANNIE and WATCHMAN.
DUNE is a proposed long-baseline neutrino experiment in which a beam of muon neutrinos will be sent from Fermilab to the Sanford Underground Research Facility in Lead, South Dakota (former home of the famous Ray Davis solar neutrino experiment).
The DUNE far detector is planned to contain 68 kilotons of liquid argon, making it much smaller than Hyper-Kamiokande. However, liquid argon provides a very high-resolution detector, allowing events to be recorded with exquisite precision. At lower energies, DUNE will be most sensitive to electron neutrinos, and Hyper-K to electron antineutrinos, providing excellent complementarity in the event of a once-in-a-lifetime opportunity such as a Galactic supernova.
In addition to our neutrino programme, we are also involved in the MICE (Muon Ionisation Cooling Experiment) project. MICE is a feasibility study for a possible future muon storage ring, which could provide a very well-defined neutrino beam with extremely well-known flavour composition, suitable for high-precision measurements of oscillation parameters or neutrino interactions.